1. Field of the Invention
The present invention relates to a method and an apparatus for alkaline saponification of a polymer film, in particular, to a method and apparatus for alkaline saponification of a cellulose ester film, which is used as a transparent support for a continuous optical compensation sheet, in which wastewater used for a water-washing step after alkaline saponification was reused, whereby environmental load as well as costs are decreased.
2. Description of the Related Art
In recent years, there have been increasing demands for optical films. Such optical films include optical compensation sheets, which are used in liquid crystal cells as a retardation plate.
When producing an optical compensation sheet by forming an orientation film and an optically anisotropic layer, in which liquid crystal molecules are fixed, on a transparent support, good adhesion is required between the transparent support (usually, a cellulose ester film represented by a cellulose acetate film) and the orientation film (usually, a polyvinyl alcohol). Since the affinity between a cellulose ester film and a polyvinyl alcohol is weak, stripping or cracking often occurs at the interface thereof; thus, a gelatin undercoating layer has been provided on the cellulose ester film. However, to develop adhesion between the undercoating layer and the cellulose ester film, a solvent that penetrates into the cellulose ester film (e.g. a ketone solvent) must be used as a solvent for the coating fluid for providing a gelatin undercoating layer. The use of such a solvent causes the swelling of a cellulose ester film, which gives rise to a problem of causing fine bending of the film in the course of shrinkage in a subsequent drying step. It was found that if an orientation film and a liquid crystal molecule layer are provided on such a bent film, thickness non-uniformity of the orientation film and liquid crystal molecule layer or orientation non-uniformity of the liquid crystal molecules occurs along the bent shape, resulting in inferior image quality of liquid crystal displays.
On the other hand, as a commonly used method for improving the adhesion between a cellulose ester film and a hydrophilic material (e.g. orientation film) without providing a gelatin undercoating layer, a method is known in which a cellulose ester film is immersed in an alkaline aqueous solution, that is, a so-called saponification bath treatment is conducted. The details of such a saponifying method are described in Japanese Patent Application Laid-Open No. 8-94838. In this saponification bath treatment, in which a cellulose ester film is immersed in an alkaline aqueous solution, both sides of the cellulose ester film undergo hydrophilization simultaneously; thus, a problem occurs, when the cellulose ester film having a hydrophilic layer of polyvinyl alcohol etc. provided on its one side is wound up into a roll, of allowing the two sides of the film to adhere to each other. There is a method, as one example of methods for hydrophilizing one side of the cellulose ester film alone in the saponification bath treatment, in which the cellulose ester film is subjected to saponification treatment with its one side, on which saponification treatment is not intended, made waterproof with a laminate or the like. However, such methods not only increase the number of troublesome steps, but also produce unnecessary wastes, and thus, not being preferable from the viewpoint of productivity and environmental conservation.
In these circumstances, a saponification method was proposed in which a solution of alkali in water or in water and an organic solvent is continuously coated on one side of the polymer film alone on which an orientation film is to be provided, and after the reaction time has elapsed, the alkaline solution is washed away from the polymer film.
There have been used various types of coating apparatus for coating a continuously traveling belt-like support (web) with such an alkaline solution. Generally, a coating section is divided in two separate sections: a section where a coating solution is transferred to a film (an application system); and a section where the coating solution transferred to a film is measured to a desired amount of coating (measuring system). Following this, coating methods are classified according to the difference in application system or in measuring system.
Coating methods classified according to the difference in application system include: for example, roller coating, dip coating and fountain coating. Coating methods classified according to the difference in measuring system include: for example, air-knife coating, blade coating and bar coating (or rod coating). There are other coating methods, in which an application system and a measuring system are included in one section, such as extrusion coating, bead coating and curtain coating.
Of the above coating methods, bar coating is a coating method in which an excess amount of coating solution is transferred to a web and the excess coating solution is scraped away by a stationary or rotating bar to adjust the amount of coating. And a wire bar, a bar having a wire wound around its surface, or a rolling bar, a bar having grooves cut in its surface is commonly used because of its coating precision. However, a wire bar presents the problem of the end of the wire, particularly the portion at which a wire is fixed, being susceptible to corrosion due to the coating environment that contains acid or alkali.
As a device which solves the above problem, a method is described in Japanese Patent Application Laid-Open No. 6-226189 which employs spot welding, adhesive or brazing to fix a wire to a bar, thereby preventing the corrosion of the soldering at the end of the wire bar. In the meantime, a method is also described in Japanese Patent Application Laid-Open No. 8-323273 in which the end of a wire bar is notched to keep the solder filling at the wire-fixed portion to a minimum.
However, use of conventional bar coater (a wire bar or a rolling bar) makes the components extracted from the film by saponification reaction or the components of the saponifying coating solution itself likely to accumulate on the groove shoulder portion of the bar, and after about 1500-minute continuous coating, the contaminant components having accumulated on the groove shoulder portion of the bar come loose and fall and stick to the film, causing coating defects. This presents the problems of causing bright spot defects in the resultant product. Besides, every time the contaminant components accumulate on the bar, the production line needs to be stopped to clean the bar, and moreover, a large amount of washing water is required for the cleaning.
The present invention has been made in the light of these circumstances. Accordingly, the object of the present invention is to provide a method and apparatus for the alkaline saponification of a polymer film which enable the saponification reaction solution on the polymer film to be uniformly and efficiently scraped away in the reaction terminating step, thereby protecting the occurrence of scratch defects of a polymer film, which might be the cause of product defects such as bright spot defects.